Chapter 3
Here you practise reading naphtha not by a single boiling point but by "the range from where it starts boiling to where it finishes boiling."
Entry point: high-school chemistry equations and mole ratios
The temperature at which distillate first starts coming over.
The temperature at which the last of the distillate finishes coming over.
The temperature span from where a mixture starts boiling to where it finishes boiling.
The range, from where to where, that is carved out as a product.
Naphtha on the light side, with more C5 / C6.
Naphtha on the heavier side, with more C7 and above.
Because naphtha is a mixture, we look at the range from IBP to FBP in distillation — a single temperature cannot describe it.
"Raising" the IBP means components that would distil below that temperature — the very light, highly volatile, easily flammable components — drop out of the cut. "Raising" the FBP means distillation continues to higher temperatures, pulling more of the heavy side in. In short, IBP is the boundary on the light side and FBP is the boundary on the heavy side.
For this course, reading "which way does it move?" is enough. Rigorous equipment conditions belong in operational documents.
Real distillation curves (volume distilled on the x-axis, temperature on the y-axis) do not always rise smoothly. Two shapes show up often:
A near-flat segment of the curve at one temperature band, indicating components clustered around that temperature.
A late, steep rise — or a "drawn-out tail" — indicating a small amount of high-boiling material still left on the heavy end.
This course does not dig deeper, but knowing "the shape itself carries hints about composition" prevents confusion when you finally see real distillation data.
Rather than treating full-range naphtha as a single thing, splitting into a light side and a heavy side makes the downstream fit much clearer.
| Type | Tends to contain | Visible uses |
|---|---|---|
| Light naphtha | More C5 / C6 | Front-end processing, isomerization, steam cracking |
| Heavy naphtha | More C7 and above | Reforming, aromatics production |
A handy mnemonic: light = evaporates easily; heavy = pairs well with reforming.
Pulling the cut toward the light side drops the average carbon number and raises volatility. Pulling it toward the heavy side does the opposite — average carbon number rises and volatility drops.
Average carbon number drops; it evaporates more readily.
Average carbon number rises; it evaporates less readily.
Even this single change significantly reshapes which downstream process the stream fits.
The name "naphtha" alone does not pin down the actual stream. Which crude, which cut, what pretreatment — any of those can shift the boiling range and composition.
That is why, in design and procurement, boiling range, composition, SDS, and analytical data come before the name.
Five questions to check how you read IBP / FBP and light / heavy.
What do the initial boiling point (IBP) and final boiling point (FBP) represent?
Answer: A
IBP is the temperature where distillation starts; FBP is where it finishes.
If you shift the cut toward the light side, what happens to the average carbon number?
Answer: A
Because you keep more of the light components, the average carbon number drops.
Which of these is most naturally fed to reforming?
Answer: A
Heavy naphtha is the natural feedstock for reforming.
Keeping composition constant, what happens to relative volatility if you pull more of the heavy side into the cut?
Answer: B
More heavy components generally means lower volatility.
Which statement correctly explains why actual naphtha streams differ even with the same name?
Answer: A
Change the crude and the cut, and composition and properties move with them.